Catalyst



Patented Mar. '22, 1932 UNITED STATES 'PATENT. ori-"ica CLARK W. DAVIS, OF SWAB'I'HMOBE; PENNSYLVANIA, ASSIGNOR '10 E. I. DU PON'I. DE NEMOURS & COMPANY, OF WILMINGTON, DELAWARE, A CORPORATION OF nmwnau CATALYST No Drawing. Original application filed February 18, 1928,. Serial F0. 255,503. Iatent Ro. 1,706,065.

Divided and this application flledjlarch 14, 1929. Serial No. 347,194.

This invention relates to a process for oxidizing ammonia to form oxides of nitrogen and more particularly it relates to an improved catalyst for carrying out such processes. This application is a division of my copending application which has terminated,

in Patent 1,706,055.

It has been known for many years that when a mixture of ammonia and air is brought into contact with a catalyst such as platinum, at a red heat, the ammonia is X1- dized forming oxides of nitrogen, water and nitrogen. Platinum, as a catalyst for this reaction, was discovered in 1839 by Kuhlmann. This metal has been used for a catalyst during the existence of this process in a specially refined state. Landis investigated the effect of various impurities i i-platinum on the conversion of ammonia and found iridium to be deleterious. This work led to his patent U. S. 1,193,799, which claims the use of iridium free platinum. The analyses of the platinum gauzes used at Muscle Shoals nitrate plant are given by Perley, J. Ind. Eng. Chem. vol. 12 (1920) page 10. The impurities shown vary from .05% to 1.08%, which it is believed must be taken into .con-

sideration as impurities which are likely to occur in platinum. To'secure high efficiency of conversion, it is necessary to-maintam the contact agent at temperatures in excess of 700 C. At temperatures above 850 C. the loss of platinum becomes more rapid but still the higher the temperature the higher is the conversion of ammonia to nitric oxide.

An object of this invention is-the production of a catalyst capable of operating at high temperatures.

A further object of theinvention is the production of a catalyst composed of-an alloy capable of loweringthe amount of loss of the catalyst at operating temperatures.

Astill further object of the invention is to increase the amount of ammonia converted at any given temperature above that obtained with essentially pure platinum in am monia oxidation, and in the oxidation of other gases including sulfur dioxide.

Other objects will appear as the description proceeds;

By incorporating larger amounts of other given temperature above that obtained with essentially pure platinum. This alloy may comprise as low as 2% andas high as 50% by weight of rhodium, or even higher. The higher percentages of rhodium have been found beneficial, but the hardness and brittleness of the alloy makes it unsuitable for working into catalytic form. It will be understood, of course, that this invention applies not only to alloys of platinum and rhodiuin .but to an alloy of platinum with any other platinum metal or metals.

In some cases, particularly where the percentage of the metal alloyed with platinum is high, the alloy will be found to be too brittle for fabrication into the usual wire gauze form. In such cases ably in the form of perforated plates.

In carrying out. my invention, I prefer to use a gauze woven from wire .003- inch in diameter, having 80 meshes per inch, and consisting of four layers as close together as possible; preferably welded together provided the meshes are not destroyed. This gauze can be'used as four flat layers as is the practice with the United Alkali Comhas no eflect on the efficiency provided the gauze temperaturecan be maintained. In

the catalyst is preferpanys converter as described in J. Soc.

. of reaction, is approximately 7 20 (3.; consem 1020 C., it would be tures of-a roximatel pp the form of preheat,

necessary to supply, in 300C. so as to get the perature. Pract1cally,-it is necessary to supply a little additional heatto. compensate for osses.

The magnitude of conversion with either a platinum gauze or a. platinum alloy gauze seems to de end on the catalyst temperature instead of t e type of converter used. If the catalyst is maintained at the same temperature in two differently designed converters v and the ammonia rating kept comparable,

the conversions are usually identical.

' In carrying out my process I may oxidize the ammonia at atmospheric pressure or I may operate the process at pressures substantially above atmospheric pressure. latter process is described by Maxted in his British Patent 127,845. There is a great advantage in operating such a process above atmospheric pressure, because the volume of the apparatus is greatly reduced, due primarily to the increase in rate of oxidation of NO to NO under increased pressure. In operating under increased pressure, it is necessary to use an ammonia-air mixture under by volume, since such mixtures support combustion more readily under pressure;

I have also found that my process may be operated at decreased pressures, that is, pressures less than atmospheric, but I pref= erably operate at pressures in the range of to 200 pounds. V

The following example is given by way of illustration as one embodiment of my invention which is applicable to a platinum-rhodium gauze catalyst, but it is to be understood, of course, that the invention is not to be limited thereby.

Burning 100 pounds of ammonia per ounce of catalyst per twenty-four hours as a 9.5% by volume'mixture, the conversions at approximately 1000 C. catalyst temperature with essentially pure platinum are approximately 6% higher at atmospheric pressure than when running under 100 pound gauge pressure. Under the same conditions, a catalyst containing approximately 90% platium and 10% rhodium gives substantially higher conversions both creased pressures. The loss of catalyst inquently, in order to obtain catalyst tempera? desired catalyst tem- The at atmospheric and inused, the loss at v proximatelyonealf that of essentially pure platinum. In operating I prefer to use temperatures of 850 C. to .1200" C.

A further-advantage in'the use'of rhodium 'in this connection lies in the fact that-since operatingtemperature isaaph rhodium has a much lower density than plativnum the use of latinum-rhodium alloy de creases the weiglit of the catal st to be used without materially increasing t e cost. Also, platinum-rhodium allo has a somewhat higher melting point t an. platinum alone Also, since rhodium hasa lowerrate of volatilization at increased temperatures, of loss of the catalyst in the case of the platinum-rhodium alloy is less than in the case of ure platinum. I

Although I have referred'to the catalyst as being in the form of a wire gauze, be understood, of course, that the catalyst may take any other form of the type customarily employed in ammonia oxidation.

As many apparently widely different embodiments of this invention may be made without departing from the spirit thereof, it is to be understood that I do'not intend to limitmyself to the specific. embodiments thereof except as indicated in the appended claims.

Iclaim; '11. A catalyst comprising a body of platinum alloyed with more than 2 per cent of rhodium, said body providing a multiplicity of contact elements, said elements being in spaced relation.

2. A catalyst comprising a body of platinum alloyed with'2 to 50 per cent of rhodium, said body providing a multiplicity of contact elements, said elements being in spaced relation.

3. A catalyst comprising a body of platinum alloyed with 5 to 25 per cent of rhodium, said body providing a multiplicity of contact elements, said elements being in spaced relation.

4. The invention of claim 1 in which the catalyst is wire gauze.

5. The invention of claim 2 in which thecatalyst is wiregauze.

6. The invention of claim 3 in which the catalyst is wire gauze.

7 The invention of claim 1 in which the catalyst is a perforated plate.

8. The invention of claim 2 .in which the catalyst is a perforated plate.

9. The invention. of claim catalyst is a perforated plate.

In testimony whereof, I aflix my signature. CLARK 'W. DAVIS.

creases very rapidly-as the temperature is.

when platinum-10% rhodium gauze is increased above approximately 850 (1., but

a will the rate V 3 in which the 

